The present invention relates to a sensor with a movable microstructure.
Sensors with movable microstructures such as, for example, inertial sensors which can measure a physical quantity relating to a movement of the sensor and produce an output signal dependent on the quantity are used in various applications, for example, in the automotive field for monitoring various devices such as air bags, anti-slip braking systems (ABS), and active suspensions, or in other fields such as consumer electronics, computers and the like. Sensors with movable microstructures are formed on a microscopic scale in chips of semiconductor material; a sensor of this type comprises a sensitive element which can produce an electrical signal relating to the movement of a microstructure movable relative to a surface of the chip.
In sensors with movable microstructures, the sensitive element has to be suitably protected by being enclosed in a hermetic structure to ensure that it operates in a controlled environment; this allows the microstructure of the sensitive element, which has a very small mass, to move with little resistance and minimal damping so as to ensure good sensitivity of the sensor. A sensor of this type also includes a circuitry which processes the electrical signal generated by the sensitive element and which, in turn, has to be encapsulated in a suitable container or package which protects the processing circuitry from external environmental conditions, ensuring that it operates correctly.
A known technique for protecting a sensor with a movable microstructure consists in the encapsulation of the sensitive element and the processing circuitry in a hermetic, for example, ceramic or metal package; the sensitive element and the processing circuitry can thus be incorporated in the same semiconductor chip.
However., this technique is extremely expensive, resulting in a high final product cost.
A different known technique consists in the production of a hollow structure on a microscopic scale (a micro-cavity) which houses the sensitive element. This isolation method involves micromachining of a silicon or glass chip which is then connected to the chip on which the sensitive element is formed, for example, by an anodic bonding technique; this technique allows inexpensive standard plastic packages to be used for encapsulating the final product. However, the known solution described above is quite complex and expensive. Moreover, this technique does not allow the sensitive element and the processing circuitry to be incorporated in the same chip since the bonding step requires the surfaces joined to be perfectly flat (with a peak-valley roughness of the order of a hundred angstroms); generally, the plate containing the sensitive element and that containing the processing circuitry are arranged side by side and are connected electrically by means of suitable metal wires.
The sensitive elements of sensors with movable microstructures are usually made in large numbers in several identical areas of a wafer of semiconductor material which are subsequently separated by a suitable cutting operation. The cutting is generally carried out by means of a high-velocity, water-cooled, diamond-blade saw. A further disadvantage of the known sensors is that, during the drying of the water used for cooling the blade, permanent sticking or xe2x80x9cstictionxe2x80x9d of the movable microstructure to an underlying surface of the semiconductor chip may occur. This physical phenomenon renders the sensor unusable.
According to principles of the present invention, a sensor is provided having a sensitive element in a first chip of semiconductor material. The sensitive element produces an electrical signal dependent on a movement of a movable microstructure relative to a surface of the first chip of semiconductor material. The sensitive element is enclosed in a hollow hermetic structure, and a processing circuit formed in the second chip of semiconductor material is coupled to receive and process the electrical signal from the sensitive element. The hermetic structure includes a metal wall around the sensitive element that is fixed between the first chip and the second chip.
The sensor with a movable microstructure according to an embodiment of the present invention is particularly compact, simple and inexpensive. This sensor permits the optional use of a plastic, and hence extremely inexpensive, package for encapsulating the final product.
A method of producing the sensor according to another embodiment of the present invention does not require micromachining of a further silicon or glass wafer or connection thereof to the wafer on which the sensitive elements are formed. Moreover, both of the semiconductor chips used in the structure of the present invention contain active elements (the sensitive element and the processing circuitry, respectively) so that there is no wastage of material.